1. Technical Field
The invention relates to an objective lens capable of efficiently converging used light onto an optical recording medium when information is recorded or reproduced, an optical pickup device and a recording and/or reproducing apparatus for the optical recoding medium. Specifically, the invention relates to an objective lens for used in recording/reproducing a high density optical recording medium with blue light having a short wavelength, an optical pickup device, and a recording and/or reproducing apparatus for the optical recording medium.
2. Description of the Related Art
Recently, various optical recording media such as DVD (digital versatile disc) and CD (compact disc which includes CD-ROM, CD-R, and CD-RW) have been widely used in various applications. However, in response to a rapid increase in data volume, an increase in storage capacity of an optical recording medium has been strongly demanded. It has been known that a decrease in wavelength of used light of a light source and an increase in numerical aperture (NA) of an objective lens are effective to increase a storage capacity of an optical recording medium. Based on this knowledge, the blu-ray disc (herein after referred to as “BD”) having about 25 GB in a single sided single layer has been put to practical use. For the BD, light which is emitted from a semiconductor laser (for example, emitting laser light having a wavelength of 405 nm) having a short wavelength light output is used as irradiation light, and a numerical aperture is increased to be equal to or greater than 0.7. In the specifications of the BD, the numerical aperture and a thickness of a protection layer (for example, the numerical aperture (NA) is 0.85, and the thickness of the protection layer is 0.1 mm) are set quietly different from those of DVD and CD.
However, in future, a further increase in density will inevitably be demanded, but it might be hard to satisfy this demand by promoting a decrease in wavelength. This is because optical transmittance of lens materials is rapidly reduced in the range of a wavelength λ less than 350 nm and thus, it is hard to obtain sufficient optical efficiency in practice.
Another way for achieving high density is to further increase the numerical aperture of the objective lens.
Meanwhile, when a lens having a large numerical aperture (hereinafter, it is referred to as a “high NA”) is designed, a single lens structure is effective to solve problems such as an increase in process number during assembly, deterioration in production efficiency, and an increase in cost.
Also, the objective lens for recording/reproducing an optical recording medium tends to be too large in on an optical axis. Accordingly, in a high NA single lens, a thickness d on the optical axis is set to be in a predetermined range which is defined based on a relation between the thickness d and a focal length f, and so excellent image height characteristic are obtained (see JP 2001-324673 A (corresponding to U.S. Pat. No. 6,411,442) and JP 2003-5032 A (corresponding to U.S. Pat. No. 7,110,344)).
Meanwhile, the objective lens for use in recording and reproducing an optical recording medium is configured so that a servo mechanism controls a focusing operation and a tracking operation. During those operations, it is required to secure a stable operation of the optical pickup apparatus. However, JP 2001-324673 A and JP 2003-5032 A don't sufficiently consider this view point.
Then, JP 2002-100067 A relates to such an objective lens, and describes such a design that a center of gravity of the objective lens is shifted to an convex optical surface on a light source side from a boundary surface between this optical surface and an edge portion (flange portion) that is used to mount the objective lens and is formed closer to an optical recording medium side than the optical surface. This design is intended to suppress the objective lens from rolling and to ensure the stable operation of the optical pickup apparatus when the focusing control and the tracking control are performed.
The objective lens described in JP 2002-100067 A is formed so that the center of gravity of the objective lens is shifted to the convex optical surface on the light source side from the boundary surface between this optical surface and the edge portion (flange portion), which is used to mount the objective lens and is formed closer to an optical recording medium side than the optical surface. However, this condition is not defined in consideration of, for example, the case where the optical recording medium side surface of the objective lens is required to have a convex shape or the case where it is necessary to set a thickness of the edge section in the optical axis direction to be large to a certain extent. Thus, this condition does not correspond to a condition for securing the stable operation of the optical pickup apparatus, and so reduces degree of freedom in design of the objective lens.